You can also download a Minecraft Pixel Art Program. In advance, sorry I don;t have a link. With these programs, you open a picture, then it automatically converts it to Minecraft blocks. It should also tell you how much of what certain blocks you will need to make it. Good luck!

I'll admit, most of my pixel art creations were not made by me. However, some were. To get you started, download Paint.NET if you don't have it already. Then, open the picture of the image you want to make into pixel art, and zoom in as much as you can. You should be able to see the pixels. Use the Minecraft blocks which look closest to that color and create your very own pixel art! I use Photoshop CS5, but Paint.NET works too.